1. Field of the Invention
This invention pertains to the production of ultra high purity polycrystalline silicon by the pyrolysis of silane. More specifically, the present invention relates to an improved process for producing such ultra high purity polycrystalline silicon which process provides for a decrease in the amount of silicon powder production leading to an increase in production capacity and electrical power efficiency.
2. Discussion of Related Art
Polycrystalline rods are primarily used as precursors for making single crystal rods for the semiconductor industry by either the float zone melting process or by the Czochralski crystal pulling technique. These single crystal rods are then processed to form silicon wafers from which silicon chips are made.
Generally, polycrystalline rods are made by the pyrolytic decomposition of a gaseous silicon compound, such as silane or a chlorosilane on a rod-shaped, heated filament made preferably from a silicon seed rod or, alternatively, from a highmelting point metal having good electrical conductivity such as tungsten or tantalum. The principles of the design of present state-of-the-art reactors for the pyrolysis of silane and chlorosilanes are set forth in, for example, U.S. Pat. Nos. 3,147,141, 4,147,814, and 4,150,168. It is generally more desirable to prepare the polycrystalline silicon rods by silane pyrolysis so as to avoid the complications caused by the formation of chloride by-products when pyrolyzing chlorosilanes. However, the silane pyrolysis is not without its own difficulties as well.
The existing reactors for the production of polysilicon rods by the pyrolysis of silane are designed as closed vessels which totally contain the reactants within closely controlled confines. This is done to minimize the potential for contamination of the hot growing silicon rods. The emphasis has been to let the reactant gases come into contact with only cooled, non-contaminating surfaces, except for the hot growing silicon rods. Reactors for the pyrolysis of silane (as compared to chlorosilanes) are characterized by having extended internal surfaces which may have water-cooled jackets to assist in cooling the reactant gases. Reactors for the pyrolysis of chlorosilanes, however, usually rely only upon the cooling obtained by the walls of the containment vessel, as taught in U.S. Pat. No. 4,147,814.
The pyrolysis of silane to form silicon and hydrogen is performed in a reactor consisting of a series of electrically heated filaments, generally silicon rods, surrounded by water cooled surfaces. The process is started with the silicon rod at ambient temperature.
The polycrystalline silicon is produced by heterogeneous decomposition of the silane on the glowing hot silicon rod. The reaction deposits silicon on the surface of the rod and releases hydrogen gas which is removed by the natural thermal convective transport of the resulting silane/hydrogen mixture leaving the reactor. This natural thermal convection is created by the hydrogen being heated by the vertically mounted hot silicon rod causing it to rise at a modest velocity estimated at about 152.4 cm./second and then cooled at the adjacent cooled reactor walls as it flows downwardly at a reduced velocity.
The silane pyrolysis proceeds at a rate determined by its concentration and local temperature. Desirable formation of dense crystalline silicon occurs in a narrow thermal boundary layer immediately surrounding the hot silicon rod. Such boundary layers are discussed in Transport Phenomena by R. B. Bird, et al. pages 366-369, (John Wiley & Sons, Inc., N.Y. 1960). To the extent that the bulk gas temperature is high enough and the distance from the rod large enough to prevent deposition of the silicon onto the rod, a competing homogeneous silane decomposition reaction takes place wherein deleterious submicron silicon powder is formed. In practice, it is found that this formation increases with increasing rod diameter due primarily to the increasing hot surface area of the glowing rod which, in turn, raises the temperature within the reactor causing mor homogeneous decomposition to take place.
Much of the powder particles are deposited on the cold reactor walls by thermophoretic deposition. However, this is a very weak process; a typical particle will recirculate many times before depositing on a cold-surface. Consequently, the number of particles in the internal recirculating gas eventually builds up to intolerable levels. This is detrimental since particles in the heated boundary layer adsorb thermal radiation further increasing thermal boundary layer temperatures and thereby the bulk gas temperature which accelerates still further homogeneous decomposition. Some of this powder ultimately deposits directly onto the silicon rod or, alternatively, flakes of deposited powder fall from the reactor walls onto the product. This affects both the surface morphology of the rod and causes undesirable contamination of the silicon rod product due to the incompatible melting behavior of the powdered silicon during subsequent single crystal formation techniques. If the powder concentration is sufficiently high, the surface morphology and/or the processability of the resulting silicon rod deteriorates to the point where the product becomes totally unacceptable.
Accordingly, the formation of silicon powder by homogeneous decomposition effectively places an upper limit on the diameter of the rods that may be grown during any production run and/or their corresponding rate of growth although larger rods at a maximum rate of growth would clearly be more cost effective. Once the amount of powder deposited on the reactor walls is such that it appears imminent that it is about to flake off, the reaction must be stopped so as to prevent contamination. So too, while it is known that increasing the silane concentration within the reactor will effect a higher growth rate of the silicon rod, the concomitant increase in the production of silicon powder negates any advantage that such increased growth rate may have.
This limited capability of the reactors translates into low productivity per reactor, a large consumption of electrical power per reactor, a large consumption of electrical power per unit of production and a large capital investment per annual unit of capacity. While the fraction of silane decomposing homogeneously into fine powder (about 1.0%) is small compared to the fraction decomposing heterogeneously into useful product, it is clear that the formation of even this relatively small amount of powder is quite detrimental and necessitates a need for finding a way to improve reactor productivity and efficiency.